WATCH: Debra Tate has been fighting to keep Van Houten, a member of the murderous "Manson family," behind bars.

Sharon Tate's sister Debra Tate is speaking out against an upcoming film starring Hilary Duff, who will portray the late actress.

Duff posted a photo to Instagram on Tuesday with the caption, "Had the incredible opportunity of playing Sharon Tate the past two weeks in an independent movie. She was an amazing woman and it was a true honor."

Hilary Duff on the streets of Los Angeles, Jan 3, 2018.

But Debra Tate doesn't feel like her sister, who was killed in 1969 at the age of 26, is being honored at all by the upcoming film, "The Haunting of Sharon Tate."

"It doesn’t matter who it is acting in it -- it’s just tasteless. It’s classless how everyone is rushing to release something for the 50th anniversary of this horrific event," she told People magazine.

A request for comment from Duff was not immediately returned to ABC News.

Sharon Tate was murdered almost five decades ago when Charles Manson and his followers stabbed the pregnant actress and a few of her friends to death at her home on Aug. 9, 1969. Manson himself died this past November while still incarcerated and Debra Tate has said that his death brought her and her family no closure at all.

"People are saying that this should be some kind of relief, but oddly enough, it really isn't," she told ABC News last year.

In the new People magazine interview, the still-grieving sister also slammed the plot of the film, specifically a part in which Sharon Tate has a premonition that she's going to die.

"I checked with all of her living friends. None of her friends had any knowledge of this. Tacky, tacky, tacky,” she calls the plot point. “It’s a total fabrication.”

Most of all, Debra Tate said, "It would have been nice if someone had contacted me."

A request for comment from Skyline Entertainment, the company reported to be producing the film, was not immediately returned to ABC News.